1. Field of the Invention
This invention relates generally to high power filters for damping resonant harmonic voltage or current components in an a.c. network and more particularly for high power filters used in conjunction with static var generators applied to an a.c. network.
2. Description of the Prior Art
The use of static var generators in a.c. networks for power factor correction or system stability can sometimes result in an increase in harmonic currents due to resonance between the capacitors and the a.c. supply lines. In a resonant condition the magnitude of the resonant harmonic component (either voltage or current) may be several times larger than their normal harmonic values. These excessively high harmonic voltages or currents may result in serious damage to electrical equipment in the a.c. network.
Conventional filters usually consist of series resonant branches connected across the power lines. The filter and remaining ac system form a LCR netowrk which is characterized by "zeros" (low impedances) and "poles" (high impedances) at frequencies where series and parallel network resonances occur. The location of the dominant "zeros" are set by design in accordance with the principal harmonics to be filtered (2nd, 3rd, 4th, 5th) and are determined by the series resonance of the filter branches. The "poles" are dependent upon the filter and ac system and occur at intermediate frequencies. The magnitude of the poles is determined by the network resistance (losses) at the "pole" frequencies.
It is generally desirable that the magnitude of the impedance "poles" be low to prevent transient disturbances of the system from causing poorly damped oscillations at the "pole" frequencies. These oscillations would manifest themselves by oscillatory currents flowing in the filter network and ac system with corresponding voltages apparing on the utility bus. The oscillatory voltages on the utility bus can cause light flicker and other electrical disturbances to consumers supplied from the source utility bus. In large static var generator systems, it is also important that the "poles" be low to damp oscillations and thus to minimize ratings of the capacitors used in the filter network.
The magnitude of the impedance "poles" may be reduced by the addition of a damping resistor, normally connected either in series or parallel with the branch inductor, however, this produces several undesirable effects. Namely, (1) the "zero" impedance of the branch is increased, thereby reducing the attenuation of the desired harmonic voltage and (2) additional losses are introduced. These losses are due to a fundamental current component as well as current components produced at the parallel resonant "pole" frequencies flowing through the damping resistor.
The fundamental component of losses produced in resistive damping is particularly bothersome since it contributes nothing to the reduction of the "pole" impedances, but does consume real power from the utility system for which the equipment operator is charged. In higher order harmonic branches (4th and above) a moderate degree of reduction in the impedance poles can be achieved without excessive fundamental dissipation in the damping resistor. In lower order harmonic branches however moderate damping produces prohibitively high losses.
Numerous techniques are available for reducing the fundamental component of losses in the damping resistor while maintaining a low "zero" impedance of the filter branch. These techniques generally employ additional series resonant sections connected in parallel with the damping resistor as shown in FIG. 1 to, first, bypass the fundamental component of current, I.sub.FM, thereby eliminating the fundamental component of losses in the resistor R.sub.N and, second, bypass the harmonic current, I.sub.N, further reducing losses and lowering the "zero" impedance of the branch. While those techniques reduce losses and improve filtering, they are expensive, particularly in high power applications since additional LC sections are required.